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Low-temperature induced phase transitions in BaWO4:Er3+ microcrystals: A Raman scattering study
Journal of Molecular Structure 1204 (2020) 127498
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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc
Low-temperature induced phase transitions in BaWO4:Er3þ microcrystals: A Raman scattering study I.P. Carvalho a, R.B. Sousa b, J.M.E. Matos b, J.V.B. Moura c, *, P.T.C. Freire d, G.S. Pinheiro a, C. Luz-Lima a ^nio Portella, Bloco 03, Universidade Federal do Piauí, CEP 64049-550, Teresina, PI, Brazil Departamento de Física, Campus Ministro Petro ^nio Portella, Bloco 02, Universidade Federal do Piauí, CEP 64049-550, Teresina, PI, Brazil Departamento de Química, Campus Ministro Petro c ~o de Materiais, Centro de Ci^ rio de Caracterizaça Laborato encias e Tecnologia, Universidade Federal do Cariri, CEP 63048-080, Juazeiro do Norte, CE, Brazil d , C.P. 6030, CEP 60455-760, Fortaleza, CE, Brazil Departamento de Física, Campus do Pici, Universidade Federal do Ceara a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 November 2019 Received in revised form 27 November 2019 Accepted 28 November 2019 Available online 29 November 2019
The synthesis of rare-earth-doped barium tungstate has increased in recent decades due to the attractive electrical and optical properties of the material. In this study, erbium-doped barium tungstate was synthesized by the co-precipitation method with three different concentrations: Ba(1-x)ErxWO4 (where x ¼ 0.00, 0.01, and 0.02). The materials were characterized by X-ray diffraction (XRD), Raman and Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and energydispersive X-ray spectroscopy (EDS). Subsequently, the samples were subjected to temperature variations in the range of 123e293 K, and their Raman spectra were collected to observe the structural changes induced by Er doping. We observed that at low temperatures, there was no indication of phase transition for pure tungstate (BaWO4); however, for doped tungstates, changes were observed in the Raman spectra, showing an unexpected structural phase transition. © 2019 Elsevier B.V. All rights reserved.
Keywords: Barium tungstate Phase transitions Raman spectroscopy Rare earth
1. Introduction In recent years, the study of nanostructured materials and their applications has significantly increased. In the area of ceramic materials, there is great interest in synthesis that improves the control of grain size and morphology. There is also interest in its wide range of applications, such as catalysis, luminescence, electronics, medicine, pigments, and cosmetics [1e10]. Among ceramic materials are those formed from tungsten and molybdenum, in particular, molybdates and tungstates with the general formula ABO4 (B ¼ Mo, W) that crystallize in two structures: wolframite (A ¼ Fe, Mn, Co, Ni, Mg, Zn) and scheelite (A ¼ Ca, Ba, Pb, Sr) [1e10]. Of particular interest is scheelite-type barium tungstate (BaWO4), space group I41/a and Z ¼ 4, which has been reported in recent studies as a material with potential for application as a host matrix of rare earth dopants (lanthanides) [3,6,7,9]. Ambast et al. [5] investigated the influence of Dy3þ (dysprosium) and Sr3þ (strontium) dopants in a photoluminescence study, obtaining blue
* Corresponding author. E-mail addresses: victor.moura@fisica.ufc.br, (J.V.B. Moura). https://doi.org/10.1016/j.molstruc.2019.127498 0022-2860/© 2019 Elsevier B.V. All rights reserved.
[email protected]
emission in the 400e450-nm spectral region attributed to charge transfer from oxygen to the metal (O2 -W6þ) for the pure material and a strong yellow emission for the dysprosium-doped material (10 mol) located at 575 nm. The researchers thus concluded that barium tungstate is suitable for light emitting diode applications. Several studies were also conducted on BaWO4 under high pressure. Fujita et al. [11] conducted a pressure study on scheelitetype compound BaWO4 (tetragonal phase) and obtained a monoclinic compound for high pressures. The high-pressure phase has eight molecular formulas per unit cell (Z ¼ 8), with P21/n being the possible space group of the new phase. As there was no report of this phase in the literature, it was named BaWO4-II. Studies of BaWO4 under high pressure using Raman spectros n et al. [12] and by copy were conducted up to 7.5 GPa by Manjo Panchal et al. [13] in the range of 1 atme16 GPa. Panchal et al. [13] also conducted an in situ study of BaWO4 by angle dispersive X-ray diffraction (ADXRD) in the same pressure range, identifying highpressure phases. These studies demonstrated a structural phase transition at pressures above 6.9 GPa due to the appearance of the fergusonite phase. With increasing pressure, a new structural phase transition was observed at 14 GPa, with subsequent amorphization at pressure values above 20 GPa.
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In the ADXRD experiments performed by Panchal et al. [13], it was difficult to identify crystallinity in the diffraction patterns above 14 GPa, indicating a breakdown in the translational symmetry of the BaWO4 system, pointing to possible pressure-induced amorphization. Upon decreasing the pressure, the original scheelite structure was recovered at approximately 3.0 GPa, indicating the reversible nature of the phase transformations. Using computer simulation (MOLDY program), Prabhatasree et al. [14] obtained n et al. [12] and Panchal results that agreed with those of Manjo et al. [13]. They demonstrated the occurrence of a phase transition at approximately 7.1 GPa from the scheelite tetragonal phase to the monoclinic fergusonite phase. At above 45 GPa, amorphization of the sample was observed. Unlike investigations of BaWO4 under high pressure, studies of BaWO4 under temperature variation are lacking. The only study examining temperature was conducted in 1984 by Desgreniers et al. [15]. The work involved subjecting samples of scheelite-type calcium (CaWO4), strontium (SrWO4), and barium (BaWO4) tungstates to low temperatures in the range of 10e300 K. Analyzing the variation rates n(T,V) of BaWO4, Desgreniers et al. [15] concluded that although there was a relationship between low-temperature variation and high-pressure application, no new structure (phase transition) at low temperatures was observed, in contrast to the results of scheelite-type BaWO4 studies at high pressures [11e14,16]. Therefore, it is important to study this material in the presence of dopants to observe the dopants’ influence on its structural and vibrational properties. The objective of this study was to examine the vibrational properties of erbium-doped BaWO4 at concentrations of 1% and 2% with low-temperature variations in the range of 123e293 K. The investigation revealed a reversible phase transition in the scheelite doped structure with characteristics similar to certain modifications observed at high pressures. 2. Experimental procedures 2.1. Synthesis of Ba(1-x)ErxWO4 microcrystals Pure and erbium-doped barium tungstates were obtained by the co-precipitation method at room temperature using deionized water as a solvent to obtain pure BaWO4. Two precursor solutions were prepared in different beakers. In beaker A, 5 103 mol BaCl2$2H2O was dissolved in 50 mL deionized water, while 5 103 mol Na2WO4$2H2O in 50 mL deionized water was added to beaker B. Each solution was maintained for 10 min while being stirred independently. After the solutions were sufficiently homogenized, the solution of beaker B was gradually added to the solution of beaker A, and the instantaneous formation of a white precipitate occurred. The procedure used for the synthesis of the doped compounds is as follows. The precursor solutions were prepared in three separate beakers. In beaker A, 5 103 mol BaCl2$2H2O was added to 40 ml deionized water and subjected to stirring. The solution of beaker B was obtained by dissolving 5 103 mol Na2WO4$2H2O in 50 ml deionized water. In beaker C, 2.5 105 mol Er2O3 to 1% (or 5 105 mol to 2%) was mixed in 10 ml deionized water. Because lanthanide oxides are not water-soluble, it was necessary to dissolve the erbium oxide with HCl to obtain the Er3þ ions in solution. Several drops of HCl were then added while the system was subjected to gentle stirring and heating until it produced an approximately clear solution. After obtaining the three homogeneous solutions, the solution of beaker C was added to the solution of beaker A, resulting in 50 ml of the mixture in the beaker A. After 10 min of stirring, the solution of beaker B was slowly added to the beaker A mixture, and the
formation of white precipitates was observed. Following this process, the precipitated material was washed (centrifuged) continuously to remove Naþ and Cl ions and to neutralize the pH, initially with deionized water followed by acetone. 2.2. Characterization The samples were characterized by X-ray diffraction (XRD) using a diffractometer (model Xpert Pro MPD, PANalytical) for polycrystalline samples. Measurements were performed in an angular range 2q of 10 to 100 and a scanning step of 0.013 min1, with CuKa radiation (l ¼ 1,540562 Å) using a nickel filter and monochromator. The structural parameters of the microcrystals were obtained using the Rietveld method [17] from the crystal data available in the Inorganic Crystal Structure Database (ICSD) Card No. 291537. Rietveld refinement was performed using the general structure analysis system (GSAS) software package. Fourth-order Chebyshev polynomials were used to fit the inelastic scattering background, and peak profile analysis was performed using the modified ThompsoneCoxeHasting pseudo-Voigt profile function. Optimized parameters included the scale factor, background, sample position in relation to the goniometer, lattice parameters (a, b, c), Lorentzian width related to crystallite size (LX), profile halfwidth parameters (GU, GV, GW), peak asymmetry related to axial divergence (S/L), atomic coordinates (x, y, z), occupancy (Frac), isotropic temperature parameter (U), ratio between Ka1 and Ka2 (Ratio), and polarization of the diffraction beam (Pola). Morphological characterization of the microcrystals was performed with a field emission scanning electron microscope (FEGSEM; model Quanta-450, Fei company) equipped with an X-ray detector (model 150, Oxford) for energy-dispersive X-ray spectroscopy (EDS). Raman spectra were collected using a Bruker Senterra confocal micro-Raman spectrometer equipped with an Olympus BX50 microscope containing a 50 lens and numerical aperture of NA ¼ 0.50 connected to a charged-coupled device. Raman measurements were performed at room temperature using a 532 nm laser with an output power of 10 mW, an acquisition time of 10 s and 5 accumulations, in the spectral region of 85e1050 cm1. Temperature-dependent measurements were performed using a Linkam thermal stage THMS600 with a step of 15 K and a relaxation time of 15 min. Fourier-transform infrared (FT-IR) spectra were measured in the spectral range of 400e1350 cm1 with a spectrometer (model VERTEX 70V, Bruker Optics) using a KBr pallet in transmittance mode. The FT-IR spectrum was recorded with 180 scans with a spectral resolution of 2 cm1. 3. Results and discussion 3.1. X-ray diffraction (XRD) To determine the structure and effect of the dopant ions in the crystalline structure, the Ba(1-x)ErxWO4 samples were characterized by powder XRD. Structural parameters of BaWO4 were obtained using the Rietveld method [17] from the crystal data available in the ICSD Card No. 291537. Rietveld analysis confirmed the formation of a tetragonal scheelite-type structure with space group I41/a, containing four molecular formulas per unit cell (Z ¼ 4)) for BaWO4 [18]. Fig. 1 presents X-ray diffractograms with Rietveld refinement of the Ba(1-x)ErxWO4 samples, demonstrating a strong correlation between observed and calculated XRD patterns; no additional peak was found. The structural refinement details are listed in Table 1. The obtained lattice parameters and volume of the unit cell are similar to values reported in previous studies [7e9,12]. The atomic coordinates of Ba(1-x)ErxWO4 unit cells are also presented in Table 1.
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Lattice parameters and atomic positions obtained from Rietveld refinement were used to model the structure (Fig. 2) using the Visualization for Electronic and Structural Analysis (VESTA) program [19]. These results demonstrate that Ba and W atoms remained in their characteristic positions while O atoms exhibited distinct displacements along the atomics coordinates x, y, and z of each unit cell due to the presence of the dopant. This indicates that the insertion of erbium ions, replacing barium ions, into the structure causes distortion in tetrahedra (WO4)2 (see tetrahedra in Fig. 2). 3.2. Morphology and composition analysis Fig. 3 presents SEM images of the obtained Ba(1-x)ErxWO4 crystals, which illustrate that these samples have a micrometric size and octahedron morphology. The EDS spectrum (Fig. 3(a)) collected on the BaWO4 microcrystals demonstrates that when synthesized, the sample consists of only Ba, W, and O, and the respective EDS elementary maps for these elements confirm the quality of the samples. In addition, a weak signal of C (~0.28 keV) was observed and can be attributed to the carbon tape used as the sample support. Fig. 3(b) and (c) confirm a successful sample doping process. It can be seen in the EDS maps that the erbium ions are uniformly distributed along the microcrystals. Table 2 presents the theoretical and experimental results of the weight percentages obtained from the stoichiometry of the BaWO4 and EDS spectra, respectively. These results indicate that the experimental percentages are close to the stoichiometric values (chemical composition and atomic fractions). In addition, we observe the calculations of the ratio Ba/W (0.747, 0.762, 0.662, and 0.731) and Ba/(Wþ4O) (0.554, 0.520, 0.279, and 0.731) for BaWO4 stoichiometric and experimental values for BaWO4, Ba0$99Er0$01WO4, and Ba0$98Er0$02WO4, respectively. The results corroborate with the Rietveld analysis. 3.3. Vibrational spectroscopy 3.3.1. Raman and infrared spectroscopy at room temperature Raman spectra of the BaWO4, Ba0$99Er0$01WO4, and Ba0$98Er0$02WO4 microcrystals are presented in Fig. 4. The scheelite-type barium tungstate (space group I41/a) has 26 vibrational modes predicted by group theory analysis [12] described by (1):
GðRamanþInfaredÞ ¼ 3Ag þ 5Au þ 5Bg þ 3Bu þ 5Eg þ 5Eu
(1)
This equation demonstrates that the Raman active modes are Ag, Bg, and Eg for a total of 13 modes. Using the experimental results and calculations of density functional theory using the Vienna ab n et al. [12] assigned the observed initio simulation package, Manjo Raman modes of three phases of the BaWO4, including the tetragonal scheelite-type phase studied in this paper and presented in Table 3. Ten vibrational modes can be identified in the spectrum of Fig. 4, which differs from the group theory presented previously [12]. However, this discrepancy can be justified by the absence of two modes (55 and 81 cm1) that are in a spectral region outside the analysis range (85 cm1 to 1050 cm1), and by the absence of a third vibrational mode (Bg) that cannot be seen because it is superimposed on the peak at approximately 329 cm1 [12] (see Table 3). In addition, the displacement of peaks belonging to the doped samples in relation to the pure crystal can be attributed to the following factors: average crystal size, distortions in the (OeWeO)/(OeBaeO) groups, interaction forces between the clusters [WO4]-[BaO8]-[WO4], and the different degrees of disorder in
Fig. 1. X-ray diffraction patterns observed and calculated by the Rietveld method of (a) BaWO4, (b) Ba0.99Er0.01WO4, and (c) Ba0.98Er0.02WO4 microcrystals.
the short-range lattice [9,20]. The presence of intense and well-defined Raman modes illustrated in Fig. 4 are related to short-range ordering. There is also a vibrational mode at 953 cm1 in the doped samples. With respect to tungstates, modes in this region are characteristic of the stretching of (WO4)2. The appearance of this Raman mode is consistent with the structural refinement results, which demonstrate that one of the effects of the presence of dopants in the structure is the deformation of the tetrahedral ions (see Fig. 2). The infrared spectra of the Ba(1-x)ErxWO4 crystals at room temperature in the spectral region of 400e1350 cm1 are presented in Fig. 5. This figure displays three vibrational modes located at 795,
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Table 1 Lattice parameters, quality indicators of structural refinement (R-values), and atomic coordinates obtained by Rietveld refinement of BaWO4 microcrystals. Lattice parameters BaWO4 (I41/a, Z ¼ 4) a ¼ 5.618916 (33) Å c ¼ 12.72914 (15) Å a ¼ b ¼ g ¼ 90 V ¼ 401.887 (7) Å Ba0$99Er0$01WO4 a ¼ 5.62303 (9) Å c ¼ 12.7342 (5) Å a ¼ b ¼ g ¼ 90 V ¼ 402.637 (20) Å Ba0$98Er0$02WO4 a ¼ 5.62109 (8) Å c ¼ 12.7284 (6) Å a ¼ b ¼ g ¼ 90 V ¼ 402.175 (21) Å
R-values
Atoms
x
y
z
Rp ¼ 6.86% Rwp ¼ 8.74% R (F2) ¼ 25.81% c2 ¼ 2.640
Ba W O e
0.0 0.0 0.6343 (9) e
0.25 0.25 0.4821 (8) e
0.625 0.125 0.2930 (4) e
Rp ¼ 7.75% Rwp ¼ 9.86% R (F2) ¼ 19.16% c2 ¼ 1.047
Ba W O Er
0.0 0.0 0.6534 (27) 0.0
0.25 0.25 0.5319 (29) 0.25
0.625 0.125 0.1797 (13) 0.625
Rp ¼ 7.99% Rwp ¼ 10.30% R (F2) ¼ 14.68% c2 ¼ 1.349
Ba W O Er
0.0 0.0 0.8155 (33) 0.0
0.25 0.25 0.4948 (32) 0.25
0.625 0.125 0.1763 (14) 0.625
Fig. 2. Schematic representation of scheelite-type tetragonal unit cells corresponding to BaWO4 crystals and tetrahedron for BaWO4, Ba0.99Er0.01WO4, and Ba0.98Er0.02WO4 under atmospheric conditions.
828, and 928 cm1. The strong absorption bands located at 828 and 795 cm1 are related to two internal modes originating from antisymmetric stretching (y3) vibrations from WO4 units. The vibrational mode at 928 cm1 observed in the doped materials indicates that one of the effects of doping is the deformation of the WO4 unit, since a symmetrical mode that is not evident in the spectrum of pure BaWO4 appears in the FT-IR spectrum of the doped materials. The FT-IR analysis is thus in accordance with the XRD and Raman results. Many materials exhibit structural instability with temperature variation, and this instability brings materials into another phase. In particular, molybdates and tungstates have tetrahedra of MoO4 or WO4 in their structure, and temperature causes rotations and deformations of these units. In molybdates and tungstates, phase transitions can be observed with relative clarity. Citing one example of this class of materials, a study on sodium tungstate and sodium molybdate indicated a phase transition between 823 and 833 K in Na2WO4, and four distinct phase transitions (at 763, 783, 823, and 943 K) in the Na2MoO4 system [21].
3.3.2. Low-temperature-dependent Raman studies Fig. 6 illustrates the Raman spectra of BaWO4, Ba0$99Er0$01WO4, and Ba0$98Er0$02WO4 microcrystals in the spectral region of 85e1050 cm1 recorded from 123 to 293 K. Fig. 6(a) illustrates the pure material (BaWO4), while Fig. 6(b) and (c) illustrate the doped
samples with 1% erbium (Ba0$99Er0$01WO4) and 2% erbium (Ba0$98Er0$02WO4), respectively. Upon decreasing the temperature, the Raman spectra remained qualitatively the same in the range of 123e293 K for the pure material (see Fig. 6(a)), how the results previously described by Desgreniers et al. [15]. Desgreniers et al. concluded that although there is a relation between low temperature and high pressure, and although BaWO4 demonstrates a phase transition under high pressures [10e14,16], there is no new phase at low temperatures [15]. Raman spectra corresponding to doped microcrystals with 1% erbium (Fig. 6(b)) and 2% erbium (Fig. 6(c)) revealed the presence of several Raman modes at a temperature close to 258 K, the main modes located at 432, 472, 508, 523, and 596 cm1 (indicated by arrows). These modes were observed in the Raman spectra obn et al. [12] in high-pressure experiments. During tained by Manjo the pressure-induced phase transition undergone by the BaWO4 system, these Raman modes, similar to the results presented in Fig. 6(b) and (c), can be attributed to the monoclinic fergusonitetype phase of BaWO4. Thus, the low-temperature-dependent Raman scattering results indicate that erbium-doped barium tungstate undergoes a phase transition at 258 K from the tetragonal scheelite-type phase to the monoclinic fergusonite-type phase. The results suggest that this structural phase transition at low temperatures by the doped barium tungstates is promoted by preexisting deformations in the WO4 tetrahedra due the presence of
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Fig. 3. SEM images and EDS spectrum of individual BaWO4 microcrystal marked and the corresponding EDS elementary maps (Ba, Er, W, and O) of the BaWO4, Ba0.99Er0.01WO4, and Ba0.98Er0.02WO4 sample.
Table 2 Weight percentages obtained in the EDS spectra of BaWO4; Ba0.99Er0.01WO4 and Ba0.98Er0.02WO4, and the Ba/W and Ba/(Wþ4O) ratio. BaWO4 - stoichiometric
BaWO4 - experimental
Element
atomic weight
Atomic %
Element
Weight %
Atomic %
Carbon Oxygen Barium Erbium Tungsten
e 63.999 137.327 e 183.841
e 66.666 16.667 e 16.667
Carbon Oxygen Barium Erbium Tungsten
12.297 18.334 29.999 e 39.370
39.342 44.036 8.394 e 8.229
Ba/W Ba/(Wþ4O)
0.747 0.554
1.000 0.200
Ba/W Ba/(Wþ4O)
0.762 0.520
1.020 0.178
Ba0$99Er0$01WO4 - experimental
Ba0$98Er0$02WO4 - experimental
Element
Weight %
Atomic %
Element
Weight %
Atomic %
Carbon Oxygen Barium Erbium Tungsten
56.494 19.680 9.486 0.005 14.341
77.348 20.229 1.136 0.004 1.283
Carbon Oxygen Barium Erbium Tungsten
46.430 19.730 13.808 0.482 19.551
72.818 23.230 1.894 0.054 2.003
(Ba þ Er)/W (Ba þ Er)/(Wþ4O)
0.662 0.279
0.889 0.053
(Ba þ Er)/W (Ba þ Er)/(Wþ4O)
0.731 0.731
0.973 0.077
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Fig. 4. Raman spectra at room temperature for BaWO4, Ba0.99Er0.01WO4, and Ba0.98Er0.02WO4 samples in the spectral region of 1050e85 cm1. Each peak highlights the respective vibrational mode symmetry.
Fig. 5. FT-IR spectra of BaWO4, Ba0.99Er0.01WO4, and Ba0.98Er0.02WO4 microcrystals at room temperature in the spectral region of 1350e400 cm1.
Table 3 Vibrational modes of Ba(1-x)ErxWO4 samples with respective identification of the vibrational modes and displacement of the modes of the doped samples relative to the pure sample. BaWO4 theoretical [5] 55 81 110 145 149 209 328 329 339 348 797 823 928
BaWO4
Ba0.99Er0.01WO4
Ba0.98Er0.02WO4
101 133 151 192 332
100 132 150 191 332
100 132 150 190 332
345 353 795 831 925
345 353 794 831 925 953
345 353 794 830 925 953
erbium ions in the structure. Fig. 7 illustrates the temperature-dependent Raman spectra during reheating to evaluate the reversibility of the modifications undergone by doped materials, in which there is a disappearance of all Raman modes corresponding to the fergusonite-type phase. In addition, the relative intensities present in the Raman spectrum prior to the cooling (scheelite) are recovered, demonstrating that the structural changes undergone by Ba0$99Er0$01WO4 and Ba0$98Er0$02WO4 microcrystals are reversible. The reversibility of the structural changes undergone by the doped samples is similar to that observed in pressure-dependent experiments in a pure BaWO4 system [12,13]. In addition, the high similarity between the low-temperature-dependent Raman results for doped BaWO4 and the results of high-pressure experiments on pure BaWO4 reveals that the effect of dopant insertion on a barium tungstate structure at low temperature is similar to applying a pressure of approximately 7.1 GPa.
Identification of the modes [5] T(Bg) T(Eg) T(Eg) R(Bg) R(Ag) y2(Eg) y2(Bg) y2(Ag) y4(Bg) y4(Eg) y3(Eg) y3(Bg) y1(Ag)
4. Conclusion This study examined the effects of low temperature on the vibrational properties of pure and doped BaWO4 obtained by the co-precipitation method. The microcrystals obtained demonstrated octahedron-type morphology with a scheelite-type tetragonal structure, and the insertion of erbium ions into the structure distorted the tetrahedra units (WO4) of the structure. Raman spectroscopy experiments at low temperatures revealed the occurrence of structural changes. The effect of doping led to a lowtemperature-induced phase transition, and the initial phase was recovered after reheating. The structural phase transition at low temperatures by the doped barium tungstates is promoted by preexisting deformations in the WO4 tetrahedra caused by the erbium atoms in the structure. Thus, doped BaWO4:Er3þ at 258 K presents a similar structure to that of pure BaWO4 under high pressure.
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Temperature
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BaWO4
Intensity
123 K 143 253 258 268 293 200
400
600
-1 Wavenumber (cm )
800
Temperature
Ba0.99Er0.01WO4
1000
Intensity
123K 183 223 233 273 293 200
400
600 -1 Wavenumber (cm )
800
Intensity
Temperature
Ba0.98Er0.02WO4
1000
123K 183 223 233 273 293 200
Fig. 6. Raman spectra of Ba(1-x)ErxWO4 in the spectral region of 1050e85 cm1 recorded in situ at low temperatures (cooling). (a) BaWO4; (b) Ba0.99Er0.01WO4; and (c) Ba0.98Er0.02WO4.
400
600
Wavenumber (cm-1)
800
1000
Fig. 7. Raman spectra of Ba(1-x)ErxWO4 in the spectral region of 1050e85 cm1 as a function of increasing temperature (reheating). (a) BaWO4, (b) Ba0.99Er0.01WO4, and (c) Ba0.98Er0.02WO4. The original spectrum is recovered.
Author contribution section I. P. Carvalho and C. Luz-Lima planned, carried out the experiments and analyzed the data. R. B. Sousa and J. M. E. Matos contributed to sample preparation. J.V.B. Moura contributed to the interpretation of the XRD and Raman results. P.T.C. Freire, G.S.
Pinheiro and J.V.B. Moura authors contributed to the final version of the manuscript. C. Luz-Lima took the lead in writing the manuscript, with input from all authors. All authors provided critical feedback and helped shape the research, analysis and manuscript.
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Acknowledgments The authors thank the Brazilian agencies CNPq, CAPES, and FUNCAP for financial support. References [1] S.M. Pourmortazavi, M. Taghdiri, N. Samimi, M. Rahimi-Nasrabadi, Eggshell bioactive membrane assisted synthesis of barium tungstate nanoparticles, Mater. Lett. 121 (2014) 5e7. [2] Y. Shen, W. Li, T. Li, Microwave-assisted synthesis of BaWO4 nanoparticles and its photoluminescence properties, Mater. Lett. 65 (2011) 2956e2958. [3] C. Bouzidi, M. Ferhi, H. Elhouichet, M. Ferid, Spectroscopic properties of rareearth (Eu3þ, Sm3þ) doped BaWO4 powders, J. Lumin. 161 (2015) 448e455. pez-Solano, P. Rodríguez-Herna ndez, S. Radescu, A. Mujica, A. Mun ~ oz, [4] J. Lo n, J. Pellicer-Porres, N. Garro, A. Segura, C.H. FerrerD. Errandonea, F.J. Manjo Roca, R.S. Kumar, O. Tschauner, G. Aquilanti, Crystal stability and pressureinduced phase transitions in scheelite AWO4 (A ¼ Ca, Sr, Ba, Pb, Eu) binary oxides.I: a review of recent ab initio calculations, ADXRD, XANES and Raman studies, Phys. Status Solidi 244 (2007) 325e330. [5] A.K. Ambast, A.K. Kunti, S. Som, S.K. Sharma, Near-white-emitting phosphors based on tungstate for phosphor-converted light-emitting diodes, Appl. Opt. 52 (2013) 8424e8431. € ls€ [6] H.P. Barbosa, J. Kai, I.G.N. Silva, L.C.V. Rodrigues, M.C.F.C. Felintoc, J. Ho a, O.L. Malta, H.F. Brito, Luminescence investigation of R3þ-doped alkaline earth tungstates prepared by a soft chemistry method, J. Lumin. 170 (2016) 736e742. [7] R.B. Sousa, V.A. Nascimento, J.M.E. Matos, C.L. Lima, C.M. Santos, ~o e propriedade fotoluminescente de M.R.M.C. Santos, Síntese, caracterizaça lmio, Cer^ tungstato de b ario puro e dopado com ho amica 61 (2015) 224e235. [8] L.S. Cavalcante, J.C. Sczancoski, J.W.M. Espinosa, J.A. Varela, P.S. Pizani, E. Longo, Photoluminescent behavior of BaWO4 powders processed in microwave-hydrothermal, J. Alloy. Comp. 474 (2009) 195e200. [9] L.S. Cavalcante, F.M.C. Batista, M.A.P. Almeida, A.C. Rabelo, I.C. Nogueira, N.C. Batista, J.A. Varela, M.R.M.C. Santos, E. Longo, M.S. Li, Structural refinement, growth process, photoluminescence and photocatalytic properties of
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(Ba1-xPr2x/3)WO4 crystals synthesized by the coprecipitation method, RSC Adv. 2 (2012) 6438e6454. P.F.S. Pereira, A.P. de Moura, I.C. Nogueira, M.V.S. Lima, E. Longo, P.C. de Sousa Filho, O.A. Serra, E.J. Nassar, I.L.V. Rosa, Study of the annealing temperature effect on the structural and luminescent properties of SrWO4:Eu phosphors prepared by a non-hydrolytic solegel process, J. Alloy. Comp. 526 (2012) 11e21. Taketoshi Fujita, Shinobu Yamaoka, Osamu Fukunaga, Pressure induced phase transformation in BaWO4, Mater. Res. Bull. 9 (1974) 141e146. n, D. Errandonea, N. Garro, J. Pellicer-Porres, P. Rodríguez-Herna nF.J. Manjo pez-Solano, A. Mujica, A. Mun ~ oz, Lattice dynamics study dez, S. Radescu, J. Lo of scheelite tungstates under high pressure I. BaWO4, Phys. Rev. B. 74 (2006) 144111. V. Panchal, N. Garg, A.K. Chauhan, S.M. Sharma, High pressure phase transitions in BaWO4, Solid State Commun. 130 (2004) 203e208. P. Goel, R. Mittal, S.L. Chaplot, High pressure phase transitions in barium tungstate, J. Phys. Conf. Ser. 377 (2012). S. Desgreniizrs, S. Jandl, C. Carlon, Temperature dependence of the Raman active phonons in CaWO4, SrWO4, and BaWO4, J. Phys. Chem. Solids (1984) 1105e1109. A. Jayaraman, B. Batlogg, L.G. Van Uitert, High-pressure Raman study of alkaline-earth tungstates and a new pressure-induced phase transition in BaWO4, Phys. Rev. B. (1983) 4774e4777. H.M. Rietveld, A profile refinement method for nuclear and magnetic structures, J. Appl. Crystallogr. 2 (1969) 65. M.J. Greaney, A. Tinoco, R.L. Brutchey, Low-temperature synthesis of homogeneous solid solutions of scheelite-structured Ca1-xSrxWO4 and Sr1-xBaxWO4 nanocrystals, Dalton Trans. 44 (2015) 15042e15048. K. Momma, F. Izumi, VESTA 3 for three-dimensional visualization of crystal, volumetric, and morphology data, J. Appl. Crystallogr. 44 (2011) 1272e1276. todo da E.G. Vieira, P.A.A. Sousa, J.M.E. Matos, M.R.M.C. Santos, Síntese pelo me ~o e caracterizaça ~o estrutural do tungstato de c coprecipitaça alcio com estrutura tipo scheelita [online], Cer^ amica 59 (2013) 417e425. n.351. C. Luz Lima, G.D. Saraiva, P.T.C. Freire, M. Maczka, W. Paraguassu, F.F. de Sousa, J. Mendes Filho, Temperature-induced phase transformations in Na2WO4 and Na2MoO4 crystals, J. Raman Spectrosc. 42 (2011) 799e802.
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Low-temperature induced phase transitions in BaWO4:Er3þ microcrystals: A Raman scattering study I.P. Carvalho a, R.B. Sousa b, J.M.E. Matos b, J.V.B. Moura c, *, P.T.C. Freire d, G.S. Pinheiro a, C. Luz-Lima a ^nio Portella, Bloco 03, Universidade Federal do Piauí, CEP 64049-550, Teresina, PI, Brazil Departamento de Física, Campus Ministro Petro ^nio Portella, Bloco 02, Universidade Federal do Piauí, CEP 64049-550, Teresina, PI, Brazil Departamento de Química, Campus Ministro Petro c ~o de Materiais, Centro de Ci^ rio de Caracterizaça Laborato encias e Tecnologia, Universidade Federal do Cariri, CEP 63048-080, Juazeiro do Norte, CE, Brazil d , C.P. 6030, CEP 60455-760, Fortaleza, CE, Brazil Departamento de Física, Campus do Pici, Universidade Federal do Ceara a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 November 2019 Received in revised form 27 November 2019 Accepted 28 November 2019 Available online 29 November 2019
The synthesis of rare-earth-doped barium tungstate has increased in recent decades due to the attractive electrical and optical properties of the material. In this study, erbium-doped barium tungstate was synthesized by the co-precipitation method with three different concentrations: Ba(1-x)ErxWO4 (where x ¼ 0.00, 0.01, and 0.02). The materials were characterized by X-ray diffraction (XRD), Raman and Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and energydispersive X-ray spectroscopy (EDS). Subsequently, the samples were subjected to temperature variations in the range of 123e293 K, and their Raman spectra were collected to observe the structural changes induced by Er doping. We observed that at low temperatures, there was no indication of phase transition for pure tungstate (BaWO4); however, for doped tungstates, changes were observed in the Raman spectra, showing an unexpected structural phase transition. © 2019 Elsevier B.V. All rights reserved.
Keywords: Barium tungstate Phase transitions Raman spectroscopy Rare earth
1. Introduction In recent years, the study of nanostructured materials and their applications has significantly increased. In the area of ceramic materials, there is great interest in synthesis that improves the control of grain size and morphology. There is also interest in its wide range of applications, such as catalysis, luminescence, electronics, medicine, pigments, and cosmetics [1e10]. Among ceramic materials are those formed from tungsten and molybdenum, in particular, molybdates and tungstates with the general formula ABO4 (B ¼ Mo, W) that crystallize in two structures: wolframite (A ¼ Fe, Mn, Co, Ni, Mg, Zn) and scheelite (A ¼ Ca, Ba, Pb, Sr) [1e10]. Of particular interest is scheelite-type barium tungstate (BaWO4), space group I41/a and Z ¼ 4, which has been reported in recent studies as a material with potential for application as a host matrix of rare earth dopants (lanthanides) [3,6,7,9]. Ambast et al. [5] investigated the influence of Dy3þ (dysprosium) and Sr3þ (strontium) dopants in a photoluminescence study, obtaining blue
* Corresponding author. E-mail addresses: victor.moura@fisica.ufc.br, (J.V.B. Moura). https://doi.org/10.1016/j.molstruc.2019.127498 0022-2860/© 2019 Elsevier B.V. All rights reserved.
[email protected]
emission in the 400e450-nm spectral region attributed to charge transfer from oxygen to the metal (O2 -W6þ) for the pure material and a strong yellow emission for the dysprosium-doped material (10 mol) located at 575 nm. The researchers thus concluded that barium tungstate is suitable for light emitting diode applications. Several studies were also conducted on BaWO4 under high pressure. Fujita et al. [11] conducted a pressure study on scheelitetype compound BaWO4 (tetragonal phase) and obtained a monoclinic compound for high pressures. The high-pressure phase has eight molecular formulas per unit cell (Z ¼ 8), with P21/n being the possible space group of the new phase. As there was no report of this phase in the literature, it was named BaWO4-II. Studies of BaWO4 under high pressure using Raman spectros n et al. [12] and by copy were conducted up to 7.5 GPa by Manjo Panchal et al. [13] in the range of 1 atme16 GPa. Panchal et al. [13] also conducted an in situ study of BaWO4 by angle dispersive X-ray diffraction (ADXRD) in the same pressure range, identifying highpressure phases. These studies demonstrated a structural phase transition at pressures above 6.9 GPa due to the appearance of the fergusonite phase. With increasing pressure, a new structural phase transition was observed at 14 GPa, with subsequent amorphization at pressure values above 20 GPa.
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In the ADXRD experiments performed by Panchal et al. [13], it was difficult to identify crystallinity in the diffraction patterns above 14 GPa, indicating a breakdown in the translational symmetry of the BaWO4 system, pointing to possible pressure-induced amorphization. Upon decreasing the pressure, the original scheelite structure was recovered at approximately 3.0 GPa, indicating the reversible nature of the phase transformations. Using computer simulation (MOLDY program), Prabhatasree et al. [14] obtained n et al. [12] and Panchal results that agreed with those of Manjo et al. [13]. They demonstrated the occurrence of a phase transition at approximately 7.1 GPa from the scheelite tetragonal phase to the monoclinic fergusonite phase. At above 45 GPa, amorphization of the sample was observed. Unlike investigations of BaWO4 under high pressure, studies of BaWO4 under temperature variation are lacking. The only study examining temperature was conducted in 1984 by Desgreniers et al. [15]. The work involved subjecting samples of scheelite-type calcium (CaWO4), strontium (SrWO4), and barium (BaWO4) tungstates to low temperatures in the range of 10e300 K. Analyzing the variation rates n(T,V) of BaWO4, Desgreniers et al. [15] concluded that although there was a relationship between low-temperature variation and high-pressure application, no new structure (phase transition) at low temperatures was observed, in contrast to the results of scheelite-type BaWO4 studies at high pressures [11e14,16]. Therefore, it is important to study this material in the presence of dopants to observe the dopants’ influence on its structural and vibrational properties. The objective of this study was to examine the vibrational properties of erbium-doped BaWO4 at concentrations of 1% and 2% with low-temperature variations in the range of 123e293 K. The investigation revealed a reversible phase transition in the scheelite doped structure with characteristics similar to certain modifications observed at high pressures. 2. Experimental procedures 2.1. Synthesis of Ba(1-x)ErxWO4 microcrystals Pure and erbium-doped barium tungstates were obtained by the co-precipitation method at room temperature using deionized water as a solvent to obtain pure BaWO4. Two precursor solutions were prepared in different beakers. In beaker A, 5 103 mol BaCl2$2H2O was dissolved in 50 mL deionized water, while 5 103 mol Na2WO4$2H2O in 50 mL deionized water was added to beaker B. Each solution was maintained for 10 min while being stirred independently. After the solutions were sufficiently homogenized, the solution of beaker B was gradually added to the solution of beaker A, and the instantaneous formation of a white precipitate occurred. The procedure used for the synthesis of the doped compounds is as follows. The precursor solutions were prepared in three separate beakers. In beaker A, 5 103 mol BaCl2$2H2O was added to 40 ml deionized water and subjected to stirring. The solution of beaker B was obtained by dissolving 5 103 mol Na2WO4$2H2O in 50 ml deionized water. In beaker C, 2.5 105 mol Er2O3 to 1% (or 5 105 mol to 2%) was mixed in 10 ml deionized water. Because lanthanide oxides are not water-soluble, it was necessary to dissolve the erbium oxide with HCl to obtain the Er3þ ions in solution. Several drops of HCl were then added while the system was subjected to gentle stirring and heating until it produced an approximately clear solution. After obtaining the three homogeneous solutions, the solution of beaker C was added to the solution of beaker A, resulting in 50 ml of the mixture in the beaker A. After 10 min of stirring, the solution of beaker B was slowly added to the beaker A mixture, and the
formation of white precipitates was observed. Following this process, the precipitated material was washed (centrifuged) continuously to remove Naþ and Cl ions and to neutralize the pH, initially with deionized water followed by acetone. 2.2. Characterization The samples were characterized by X-ray diffraction (XRD) using a diffractometer (model Xpert Pro MPD, PANalytical) for polycrystalline samples. Measurements were performed in an angular range 2q of 10 to 100 and a scanning step of 0.013 min1, with CuKa radiation (l ¼ 1,540562 Å) using a nickel filter and monochromator. The structural parameters of the microcrystals were obtained using the Rietveld method [17] from the crystal data available in the Inorganic Crystal Structure Database (ICSD) Card No. 291537. Rietveld refinement was performed using the general structure analysis system (GSAS) software package. Fourth-order Chebyshev polynomials were used to fit the inelastic scattering background, and peak profile analysis was performed using the modified ThompsoneCoxeHasting pseudo-Voigt profile function. Optimized parameters included the scale factor, background, sample position in relation to the goniometer, lattice parameters (a, b, c), Lorentzian width related to crystallite size (LX), profile halfwidth parameters (GU, GV, GW), peak asymmetry related to axial divergence (S/L), atomic coordinates (x, y, z), occupancy (Frac), isotropic temperature parameter (U), ratio between Ka1 and Ka2 (Ratio), and polarization of the diffraction beam (Pola). Morphological characterization of the microcrystals was performed with a field emission scanning electron microscope (FEGSEM; model Quanta-450, Fei company) equipped with an X-ray detector (model 150, Oxford) for energy-dispersive X-ray spectroscopy (EDS). Raman spectra were collected using a Bruker Senterra confocal micro-Raman spectrometer equipped with an Olympus BX50 microscope containing a 50 lens and numerical aperture of NA ¼ 0.50 connected to a charged-coupled device. Raman measurements were performed at room temperature using a 532 nm laser with an output power of 10 mW, an acquisition time of 10 s and 5 accumulations, in the spectral region of 85e1050 cm1. Temperature-dependent measurements were performed using a Linkam thermal stage THMS600 with a step of 15 K and a relaxation time of 15 min. Fourier-transform infrared (FT-IR) spectra were measured in the spectral range of 400e1350 cm1 with a spectrometer (model VERTEX 70V, Bruker Optics) using a KBr pallet in transmittance mode. The FT-IR spectrum was recorded with 180 scans with a spectral resolution of 2 cm1. 3. Results and discussion 3.1. X-ray diffraction (XRD) To determine the structure and effect of the dopant ions in the crystalline structure, the Ba(1-x)ErxWO4 samples were characterized by powder XRD. Structural parameters of BaWO4 were obtained using the Rietveld method [17] from the crystal data available in the ICSD Card No. 291537. Rietveld analysis confirmed the formation of a tetragonal scheelite-type structure with space group I41/a, containing four molecular formulas per unit cell (Z ¼ 4)) for BaWO4 [18]. Fig. 1 presents X-ray diffractograms with Rietveld refinement of the Ba(1-x)ErxWO4 samples, demonstrating a strong correlation between observed and calculated XRD patterns; no additional peak was found. The structural refinement details are listed in Table 1. The obtained lattice parameters and volume of the unit cell are similar to values reported in previous studies [7e9,12]. The atomic coordinates of Ba(1-x)ErxWO4 unit cells are also presented in Table 1.
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Lattice parameters and atomic positions obtained from Rietveld refinement were used to model the structure (Fig. 2) using the Visualization for Electronic and Structural Analysis (VESTA) program [19]. These results demonstrate that Ba and W atoms remained in their characteristic positions while O atoms exhibited distinct displacements along the atomics coordinates x, y, and z of each unit cell due to the presence of the dopant. This indicates that the insertion of erbium ions, replacing barium ions, into the structure causes distortion in tetrahedra (WO4)2 (see tetrahedra in Fig. 2). 3.2. Morphology and composition analysis Fig. 3 presents SEM images of the obtained Ba(1-x)ErxWO4 crystals, which illustrate that these samples have a micrometric size and octahedron morphology. The EDS spectrum (Fig. 3(a)) collected on the BaWO4 microcrystals demonstrates that when synthesized, the sample consists of only Ba, W, and O, and the respective EDS elementary maps for these elements confirm the quality of the samples. In addition, a weak signal of C (~0.28 keV) was observed and can be attributed to the carbon tape used as the sample support. Fig. 3(b) and (c) confirm a successful sample doping process. It can be seen in the EDS maps that the erbium ions are uniformly distributed along the microcrystals. Table 2 presents the theoretical and experimental results of the weight percentages obtained from the stoichiometry of the BaWO4 and EDS spectra, respectively. These results indicate that the experimental percentages are close to the stoichiometric values (chemical composition and atomic fractions). In addition, we observe the calculations of the ratio Ba/W (0.747, 0.762, 0.662, and 0.731) and Ba/(Wþ4O) (0.554, 0.520, 0.279, and 0.731) for BaWO4 stoichiometric and experimental values for BaWO4, Ba0$99Er0$01WO4, and Ba0$98Er0$02WO4, respectively. The results corroborate with the Rietveld analysis. 3.3. Vibrational spectroscopy 3.3.1. Raman and infrared spectroscopy at room temperature Raman spectra of the BaWO4, Ba0$99Er0$01WO4, and Ba0$98Er0$02WO4 microcrystals are presented in Fig. 4. The scheelite-type barium tungstate (space group I41/a) has 26 vibrational modes predicted by group theory analysis [12] described by (1):
GðRamanþInfaredÞ ¼ 3Ag þ 5Au þ 5Bg þ 3Bu þ 5Eg þ 5Eu
(1)
This equation demonstrates that the Raman active modes are Ag, Bg, and Eg for a total of 13 modes. Using the experimental results and calculations of density functional theory using the Vienna ab n et al. [12] assigned the observed initio simulation package, Manjo Raman modes of three phases of the BaWO4, including the tetragonal scheelite-type phase studied in this paper and presented in Table 3. Ten vibrational modes can be identified in the spectrum of Fig. 4, which differs from the group theory presented previously [12]. However, this discrepancy can be justified by the absence of two modes (55 and 81 cm1) that are in a spectral region outside the analysis range (85 cm1 to 1050 cm1), and by the absence of a third vibrational mode (Bg) that cannot be seen because it is superimposed on the peak at approximately 329 cm1 [12] (see Table 3). In addition, the displacement of peaks belonging to the doped samples in relation to the pure crystal can be attributed to the following factors: average crystal size, distortions in the (OeWeO)/(OeBaeO) groups, interaction forces between the clusters [WO4]-[BaO8]-[WO4], and the different degrees of disorder in
Fig. 1. X-ray diffraction patterns observed and calculated by the Rietveld method of (a) BaWO4, (b) Ba0.99Er0.01WO4, and (c) Ba0.98Er0.02WO4 microcrystals.
the short-range lattice [9,20]. The presence of intense and well-defined Raman modes illustrated in Fig. 4 are related to short-range ordering. There is also a vibrational mode at 953 cm1 in the doped samples. With respect to tungstates, modes in this region are characteristic of the stretching of (WO4)2. The appearance of this Raman mode is consistent with the structural refinement results, which demonstrate that one of the effects of the presence of dopants in the structure is the deformation of the tetrahedral ions (see Fig. 2). The infrared spectra of the Ba(1-x)ErxWO4 crystals at room temperature in the spectral region of 400e1350 cm1 are presented in Fig. 5. This figure displays three vibrational modes located at 795,
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Table 1 Lattice parameters, quality indicators of structural refinement (R-values), and atomic coordinates obtained by Rietveld refinement of BaWO4 microcrystals. Lattice parameters BaWO4 (I41/a, Z ¼ 4) a ¼ 5.618916 (33) Å c ¼ 12.72914 (15) Å a ¼ b ¼ g ¼ 90 V ¼ 401.887 (7) Å Ba0$99Er0$01WO4 a ¼ 5.62303 (9) Å c ¼ 12.7342 (5) Å a ¼ b ¼ g ¼ 90 V ¼ 402.637 (20) Å Ba0$98Er0$02WO4 a ¼ 5.62109 (8) Å c ¼ 12.7284 (6) Å a ¼ b ¼ g ¼ 90 V ¼ 402.175 (21) Å
R-values
Atoms
x
y
z
Rp ¼ 6.86% Rwp ¼ 8.74% R (F2) ¼ 25.81% c2 ¼ 2.640
Ba W O e
0.0 0.0 0.6343 (9) e
0.25 0.25 0.4821 (8) e
0.625 0.125 0.2930 (4) e
Rp ¼ 7.75% Rwp ¼ 9.86% R (F2) ¼ 19.16% c2 ¼ 1.047
Ba W O Er
0.0 0.0 0.6534 (27) 0.0
0.25 0.25 0.5319 (29) 0.25
0.625 0.125 0.1797 (13) 0.625
Rp ¼ 7.99% Rwp ¼ 10.30% R (F2) ¼ 14.68% c2 ¼ 1.349
Ba W O Er
0.0 0.0 0.8155 (33) 0.0
0.25 0.25 0.4948 (32) 0.25
0.625 0.125 0.1763 (14) 0.625
Fig. 2. Schematic representation of scheelite-type tetragonal unit cells corresponding to BaWO4 crystals and tetrahedron for BaWO4, Ba0.99Er0.01WO4, and Ba0.98Er0.02WO4 under atmospheric conditions.
828, and 928 cm1. The strong absorption bands located at 828 and 795 cm1 are related to two internal modes originating from antisymmetric stretching (y3) vibrations from WO4 units. The vibrational mode at 928 cm1 observed in the doped materials indicates that one of the effects of doping is the deformation of the WO4 unit, since a symmetrical mode that is not evident in the spectrum of pure BaWO4 appears in the FT-IR spectrum of the doped materials. The FT-IR analysis is thus in accordance with the XRD and Raman results. Many materials exhibit structural instability with temperature variation, and this instability brings materials into another phase. In particular, molybdates and tungstates have tetrahedra of MoO4 or WO4 in their structure, and temperature causes rotations and deformations of these units. In molybdates and tungstates, phase transitions can be observed with relative clarity. Citing one example of this class of materials, a study on sodium tungstate and sodium molybdate indicated a phase transition between 823 and 833 K in Na2WO4, and four distinct phase transitions (at 763, 783, 823, and 943 K) in the Na2MoO4 system [21].
3.3.2. Low-temperature-dependent Raman studies Fig. 6 illustrates the Raman spectra of BaWO4, Ba0$99Er0$01WO4, and Ba0$98Er0$02WO4 microcrystals in the spectral region of 85e1050 cm1 recorded from 123 to 293 K. Fig. 6(a) illustrates the pure material (BaWO4), while Fig. 6(b) and (c) illustrate the doped
samples with 1% erbium (Ba0$99Er0$01WO4) and 2% erbium (Ba0$98Er0$02WO4), respectively. Upon decreasing the temperature, the Raman spectra remained qualitatively the same in the range of 123e293 K for the pure material (see Fig. 6(a)), how the results previously described by Desgreniers et al. [15]. Desgreniers et al. concluded that although there is a relation between low temperature and high pressure, and although BaWO4 demonstrates a phase transition under high pressures [10e14,16], there is no new phase at low temperatures [15]. Raman spectra corresponding to doped microcrystals with 1% erbium (Fig. 6(b)) and 2% erbium (Fig. 6(c)) revealed the presence of several Raman modes at a temperature close to 258 K, the main modes located at 432, 472, 508, 523, and 596 cm1 (indicated by arrows). These modes were observed in the Raman spectra obn et al. [12] in high-pressure experiments. During tained by Manjo the pressure-induced phase transition undergone by the BaWO4 system, these Raman modes, similar to the results presented in Fig. 6(b) and (c), can be attributed to the monoclinic fergusonitetype phase of BaWO4. Thus, the low-temperature-dependent Raman scattering results indicate that erbium-doped barium tungstate undergoes a phase transition at 258 K from the tetragonal scheelite-type phase to the monoclinic fergusonite-type phase. The results suggest that this structural phase transition at low temperatures by the doped barium tungstates is promoted by preexisting deformations in the WO4 tetrahedra due the presence of
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Fig. 3. SEM images and EDS spectrum of individual BaWO4 microcrystal marked and the corresponding EDS elementary maps (Ba, Er, W, and O) of the BaWO4, Ba0.99Er0.01WO4, and Ba0.98Er0.02WO4 sample.
Table 2 Weight percentages obtained in the EDS spectra of BaWO4; Ba0.99Er0.01WO4 and Ba0.98Er0.02WO4, and the Ba/W and Ba/(Wþ4O) ratio. BaWO4 - stoichiometric
BaWO4 - experimental
Element
atomic weight
Atomic %
Element
Weight %
Atomic %
Carbon Oxygen Barium Erbium Tungsten
e 63.999 137.327 e 183.841
e 66.666 16.667 e 16.667
Carbon Oxygen Barium Erbium Tungsten
12.297 18.334 29.999 e 39.370
39.342 44.036 8.394 e 8.229
Ba/W Ba/(Wþ4O)
0.747 0.554
1.000 0.200
Ba/W Ba/(Wþ4O)
0.762 0.520
1.020 0.178
Ba0$99Er0$01WO4 - experimental
Ba0$98Er0$02WO4 - experimental
Element
Weight %
Atomic %
Element
Weight %
Atomic %
Carbon Oxygen Barium Erbium Tungsten
56.494 19.680 9.486 0.005 14.341
77.348 20.229 1.136 0.004 1.283
Carbon Oxygen Barium Erbium Tungsten
46.430 19.730 13.808 0.482 19.551
72.818 23.230 1.894 0.054 2.003
(Ba þ Er)/W (Ba þ Er)/(Wþ4O)
0.662 0.279
0.889 0.053
(Ba þ Er)/W (Ba þ Er)/(Wþ4O)
0.731 0.731
0.973 0.077
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Fig. 4. Raman spectra at room temperature for BaWO4, Ba0.99Er0.01WO4, and Ba0.98Er0.02WO4 samples in the spectral region of 1050e85 cm1. Each peak highlights the respective vibrational mode symmetry.
Fig. 5. FT-IR spectra of BaWO4, Ba0.99Er0.01WO4, and Ba0.98Er0.02WO4 microcrystals at room temperature in the spectral region of 1350e400 cm1.
Table 3 Vibrational modes of Ba(1-x)ErxWO4 samples with respective identification of the vibrational modes and displacement of the modes of the doped samples relative to the pure sample. BaWO4 theoretical [5] 55 81 110 145 149 209 328 329 339 348 797 823 928
BaWO4
Ba0.99Er0.01WO4
Ba0.98Er0.02WO4
101 133 151 192 332
100 132 150 191 332
100 132 150 190 332
345 353 795 831 925
345 353 794 831 925 953
345 353 794 830 925 953
erbium ions in the structure. Fig. 7 illustrates the temperature-dependent Raman spectra during reheating to evaluate the reversibility of the modifications undergone by doped materials, in which there is a disappearance of all Raman modes corresponding to the fergusonite-type phase. In addition, the relative intensities present in the Raman spectrum prior to the cooling (scheelite) are recovered, demonstrating that the structural changes undergone by Ba0$99Er0$01WO4 and Ba0$98Er0$02WO4 microcrystals are reversible. The reversibility of the structural changes undergone by the doped samples is similar to that observed in pressure-dependent experiments in a pure BaWO4 system [12,13]. In addition, the high similarity between the low-temperature-dependent Raman results for doped BaWO4 and the results of high-pressure experiments on pure BaWO4 reveals that the effect of dopant insertion on a barium tungstate structure at low temperature is similar to applying a pressure of approximately 7.1 GPa.
Identification of the modes [5] T(Bg) T(Eg) T(Eg) R(Bg) R(Ag) y2(Eg) y2(Bg) y2(Ag) y4(Bg) y4(Eg) y3(Eg) y3(Bg) y1(Ag)
4. Conclusion This study examined the effects of low temperature on the vibrational properties of pure and doped BaWO4 obtained by the co-precipitation method. The microcrystals obtained demonstrated octahedron-type morphology with a scheelite-type tetragonal structure, and the insertion of erbium ions into the structure distorted the tetrahedra units (WO4) of the structure. Raman spectroscopy experiments at low temperatures revealed the occurrence of structural changes. The effect of doping led to a lowtemperature-induced phase transition, and the initial phase was recovered after reheating. The structural phase transition at low temperatures by the doped barium tungstates is promoted by preexisting deformations in the WO4 tetrahedra caused by the erbium atoms in the structure. Thus, doped BaWO4:Er3þ at 258 K presents a similar structure to that of pure BaWO4 under high pressure.
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Temperature
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BaWO4
Intensity
123 K 143 253 258 268 293 200
400
600
-1 Wavenumber (cm )
800
Temperature
Ba0.99Er0.01WO4
1000
Intensity
123K 183 223 233 273 293 200
400
600 -1 Wavenumber (cm )
800
Intensity
Temperature
Ba0.98Er0.02WO4
1000
123K 183 223 233 273 293 200
Fig. 6. Raman spectra of Ba(1-x)ErxWO4 in the spectral region of 1050e85 cm1 recorded in situ at low temperatures (cooling). (a) BaWO4; (b) Ba0.99Er0.01WO4; and (c) Ba0.98Er0.02WO4.
400
600
Wavenumber (cm-1)
800
1000
Fig. 7. Raman spectra of Ba(1-x)ErxWO4 in the spectral region of 1050e85 cm1 as a function of increasing temperature (reheating). (a) BaWO4, (b) Ba0.99Er0.01WO4, and (c) Ba0.98Er0.02WO4. The original spectrum is recovered.
Author contribution section I. P. Carvalho and C. Luz-Lima planned, carried out the experiments and analyzed the data. R. B. Sousa and J. M. E. Matos contributed to sample preparation. J.V.B. Moura contributed to the interpretation of the XRD and Raman results. P.T.C. Freire, G.S.
Pinheiro and J.V.B. Moura authors contributed to the final version of the manuscript. C. Luz-Lima took the lead in writing the manuscript, with input from all authors. All authors provided critical feedback and helped shape the research, analysis and manuscript.
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